1. Field of the Invention
The present invention relates to corrosion-resistant chromium steels used in welded structural elements. In particular, the present invention relates to a corrosion-resistant chromium steel suitable for architectural and civil engineering structural elements which are used in obscure places of completed structures and which are not exposed to severe environments, unlike outer walls.
2. Description of the Related Art
Traditionally, plain steels such as SS400, high tensile strength steels such as SM490, and coated or plated materials thereof have been primarily used in architectural and civil engineering structural elements.
With trends towards large constructions and a greater diversity of designs, the applications of various steels and materials have recently begun to be studied.
In particular, materials are being selected in consideration of life cycle costs (LCC) in view of growing environmental concerns. For example, a requirement for house designing is a lifetime of over one hundred years.
A possible means of prolonging the lifetime of a structure is by increasing the thickness of the plating layer of plated steel sheets. Unfortunately, a thick plated layer is not suitable in practice for architectural structures that inevitably require welding because the plated layer requires a great labor for treatment of the welded portion after welding.
In such circumstances, a possible material for architectural and civil engineering structural elements is an Fexe2x80x94Cr alloy which has high corrosion resistance, which substantially requires no maintenance expenses for rust prevention, and which can be easily recycled.
Typical chromium steels, namely, stainless steels are divided broadly into ferritic stainless steels such as SUS430, austenitic stainless steels such as SUS304, martensitic stainless steels such as SUS410, and duplex stainless steels such as SUS329.
Of these stainless steels, austenitic stainless steels excel in strength, corrosion resistance, weldability, toughness at weld portions, and versatility. Thus, attempts have been made to apply austenitic stainless steels to architectural and civil engineering structural elements.
Austenitic stainless steels, however, have the following drawbacks:
(1) The steel is extremely expensive compared with plain steels because of the high content of alloying elements such as nickel and chromium;
(2) The steel is highly susceptible to stress corrosion cracking; and
(3) The steel has a large thermal expansion coefficient and a small thermal conductivity, which cause ready accumulation of stress due to welding heat and are not suitable for the application of the steel to high-precision components.
Accordingly, it is difficult to use austenitic stainless steels in general-purpose structural elements as substitutions for plain steels or coated or plated plain steels.
Applications of low-chromium steels and in particular martensitic stainless steels to architectural and civil engineering structural elements have recently been examined as substitutions for coated or plated plain steels.
Martensitic stainless steels are exceptionally inexpensive compared with austenitic stainless steels containing large amounts of expensive nickel, have a low thermal expansion coefficient and high thermal conductivity, and have significantly high corrosion resistance and high strength compared with plain steels.
Furthermore, the martensitic stainless steels do not cause "sgr"-embrittlement and 475xc2x0 C.-embrittlement, which are inherent in high-chromium steels, and stress-corrosion cracking in chloride environments, which is inherent in austenitic stainless steels.
However, the martensitic stainless steels such as SUS410 steel have high carbon contents of about 0.1 mass percent and thus exhibit low toughness and poor workability in the weld zone. In addition, the martensitic stainless steels require preheating for welding, which results in poor welding efficiency. Thus, known martensitic stainless steels are not suitable for applications which require welding.
For example, Japanese Examined Patent Publication No. 51-13463 discloses a martensitic stainless steel for welded structural elements. This martensitic stainless steel contains 10 to 18 mass percent Cr, 0.1 to 3.4 mass percent Ni, 1.0 mass percent or less of Si, and 4.0 mass percent or less of Mn. The C content is reduced to 0.03 mass percent or less and the N content is reduced to 0.02 mass percent or less to form a massive martensitic structure at the welded heat affected zone.
Japanese Examined Patent Publication No. 57-28738 discloses another martensitic stainless steel for welded structural elements having high toughness and high workability at the weld zone. This martensitic stainless steel contains 10 to 13.5 mass percent Cr, 0.5 mass percent or less of Si, and 1.0 to 3.5 mass percent Mn. Both the C content and the N content are reduced to 0.020 mass percent or less and the Ni content is reduced to less than 0.1 mass percent to eliminate the necessity of preheating and postheating for welding.
It is preferable that the chromium content in the structural steel be higher in view of corrosion resistance. However, in general, many structural steels used do not always require significantly high corrosion resistance, for example, no rusting. In particular, structural elements which are used in obscure places of completed structures and which are not exposed to severe environments require only moderate corrosion resistance so that no rust fluid flows out, for long term use. In other words, these structural elements do not require the high corrosion resistance of known stainless steels.
Furthermore, it is preferable that hot-rolled steel sheets or descaled hot-rolled steel sheets be used in architectural and civil engineering structural elements from economical standpoint because high-quality surface properties are not necessary for these elements.
In order satisfy the above requirements, inexpensive chromium steels are currently being developed by reducing the chromium content to less than 10 mass percent under condition that hot-rolled or descaled hot-rolled steel sheets are used without further treatment.
For example, Japanese Patent No. 3039630 discloses a low-corrosion-rate steel for architectural structural elements. The steel contains 6 to 18 mass percent Cr, 0.05 to 1.5 mass percent Si, and 0.05 to 1.5 mass percent Mn. The C content is controlled within the range of 0.005 to 0.1 mass percent. The finishing delivery temperature during hot rolling is controlled to 780xc2x0 C. or less to suppress local corrosion by intentionally forming of a chromium depletion layer with a thickness of at least 5 xcexcm right below the oxide scale.
Japanese Unexamined Patent Publication No. 11-323505 discloses a steel containing 5 to 10 mass percent Cr, 0.05 to 1.0 mass percent Si, and 0.05 to 2.0 mass percent Mn. Both the C content and the N content in the steel are reduced to 0.005 to 0.03 mass percent. The Cr content at a depth in the range of 0.5 to 10 xcexcm from the topmost surface of the metal portion is reduced to less than 5 mass percent to generate uniform entire corrosion, so that a local and significant decrease in thickness is reduced. As a result, a decrease in strength and destruction due to corrosion are suppressed.
In these technologies disclosed in Japanese Patent No. 3039630 and Japanese Unexamined Patent Publication No. 11-323505, however, a low-chromium steel containing less than 10 mass percent Cr does not have a sufficient corrosion resistance. Further improvement in long-term corrosion resistance is thereby required.
Furthermore, the technology disclosed in Japanese Unexamined Patent Publication No. 11-323505 is aimed at cladding, thermal spray coating, and plating procedures. Thus, this technology has a problem of high production costs.
The present inventors have developed Fexe2x80x94Cr alloys having excellent weldability and high initial corrosion resistance without a significant increase in Ni, Cu, Cr, and Mo content, the addition of Nb and Ti, nor a marked decrease in C and N, and have filed Japanese Patent Application Nos. 2000-161626 and 2000-161627.
Specifically, the Fexe2x80x94Cr alloy contains more than 8 mass percent to less than 15 mass percent Cr, 0.01 mass percent to less than 0.5 mass percent Co, 0.01 mass percent to less than 0.5 mass percent V, and 0.001 mass percent to less than 0.05 mass percent W. Moreover, the composition is adjusted so that a X value is 11.0 or less and a Z value is in the range of 0.03 to 1.5, wherein
X valuexe2x95x90Cr+Mo+1.5Si+0.5Nb+0.2V+0.3 W+8Alxe2x88x92Nixe2x88x920.6Coxe2x88x920.5 Mnxe2x88x9230Cxe2x88x9230Nxe2x88x920.5Cu 
Z valuexe2x95x90Co+1.5V+4.8W 
Preferably, the composition is adjusted so that the ratio C/N is 0.6 or less.
However, this alloy containing a large amount of Cr has an economic disadvantage. In addition, a steel containing about 11 mass percent or more of Cr must be annealed for softening, making the economic disadvantage more marked. Although a steel containing more Cr is resistant to corrosion in long-term use, local corrosion readily occurs. Such localized corrosion is more disadvantageous to strength than uniform corrosion.
An object of the present invention is to provide an inexpensive corrosion-resistant chromium steel which has a low Cr content of less than 10 mass percent, which has a lifetime of at least 100 years in structural welding applications where excellent appearance is not required, and which can be used as a hot-rolled or descaled hot-rolled state without further treatment. Such a steel is suitable for architectural and civil engineering structural elements which are used in obscure places of completed structures and which are not exposed to severe environments.
The steel of the invention has a structure substantially composed of a single ferritic phase after hot rolling, a tensile strength TS of 400 to 550 MPa, and a decrease in strength due to corrosion of 10% or less and preferably 5% or less after use for 100 or more years as architectural and civil engineering structural elements compared with the strength before use.
Furthermore, in the steel of the invention, the heat-affected zone is substantially composed of a martensitic structure to suppress the formation of coarse grains, which cause deterioration of toughness at the weld zone.
The steel of the invention can be formed into steel pipes and section steels by welding and shaping and be used in structural elements.
According to an aspect of the invention, a corrosion-resistant chromium steel for architectural and civil engineering structural elements, comprises from about 0.0015 to about 0.02 mass percent C, from about 0.0015 to about 0.02 mass percent N, from about 0.1 to about 1.0 mass percent Si, from about 0.1 to about 3.0 mass percent Mn, more than about 5 mass percent to less than about 10 mass percent Cr, from about 0.01 to about 3.0 mass percent Ni, about 0.1 mass percent or less of Al, about 0.05 mass percent or less of P, about 0.03 mass percent or less of S, from about 0.01 to about 1.0 mass percent Co, and the balance being Fe and incidental impurities, the steel thereby having high long-term corrosion resistance and high weld-zone toughness.
Preferably, the steel further comprises from about 0.01 to about 0.5 mass percent V and from about 0.001 to about 0.05 mass percent W, the Cr content is in the range of more than about 5 mass percent to less than about 8 mass percent, and a Z value represented by formula (1) is in the range of 0.03 to 1.5:
Z valuexe2x95x90([%Co]+1.5[%V]+4.8[%W])xe2x80x83xe2x80x83(1) 
wherein [%Co], [%V], [%W], respectively, represent Co, V, and W contents by mass percent.
Preferably, the Cr content is in the range of more than about 5 mass percent to less than about 7.5 mass percent and the W content is in the range of from about 0.001 to about 0.03 mass percent.
Preferably, the steel further comprises at least one of about 3.0 mass percent or less of Cu and about 3.0 mass percent or less of Mo.
Preferably, the steel further comprises from about 0.0002 to about 0.0030 mass percent of B.